1. Field of the Invention
This invention is related to triaxial tests of rock and, in one aspect, to triaxial test apparatuses and methods for low permeability rock, e.g. shales, to determine various properties of the rock such as mechanical strength properties, ultrasonic velocities and elastic properties, e.g. Poisson's ratio, Young's Modulus, bulk modulus, etc., and elasto-plastic properties. In one aspect this invention is directed to triaxial test apparatuses and methods employing ultrasonic transducers to rapidly measure ultrasonic velocities of low permeability rocks during a triaxial test with accurate pore pressure measurements, allowing determination of an ultrasonic velocity-mean effective stress relationships with one test or multiple tests and for other relationships including ultrasonic velocity-mean effective stress-elastic property; and ultrasonic velocity-mean effective stress-elasto plastic property.
2. Description of Related Art
Wellbore stability has been frequently identified as one of the major sources of trouble costs for drilling in shale. In order to minimize wellbore stability related trouble costs, testing programs have been developed to build data bases and generalize the mechanical properties of different shales. Very high quality and useful results have been obtained. However, such test programs are very tedious and require many tests (e.g. 15-20) to quantify the properties for one shale type.
FIG. 1 illustrates a conventional triaxial test apparatus for rock testing. The test is performed inside an enclosed hydraulic cell and a confining fluid pumped into the cell provides a confining pressure on a rock sample specimen to be tested. The rock sample is placed on the bottom end cap for support. During the test, hydraulic pressure is applied around the sample to generate an isostatic confining stress and then an axial load (usually via a load piston) is applied to the top end cap to generate a deviatoric (shear) stress. The prior art does not teach monitoring of rock water content during a triaxial test of low permeability rocks. Ultrasonic transducers have been used near opposite sides of a rock sample during tests of it.
Prior art tests have employed different apparatuses and methods to obtain ultrasonic velocity measurements for different rocks. However, the prior art methods yield inaccurate results and require a relatively long time to complete measurements for a low permeability rock, such as shale. The prior art methods use badly disturbed, predried rock samples. The samples then are exposed to deionized water to achieve saturation before testing. It is well known that shales alter their mechanical and chemical reaction properties after they lose much of their water to an external system or gain water from an external water source. The prior art methods do not use the special procedures and equipment described in the applications appended hereto, and therefore, well preserved shale samples are not used and pore pressures and effective stresses are not accurately determined during tests. Applicants are unaware of any prior art disclosing triaxial test apparatuses and methods for low permeability rock, such as shales, which employ ultrasonic transducers to measure ultrasonic velocity during a triaxial test at different effective stress levels.
There has long been a need for triaxial test apparatus and methods for low permeability rock utilizing ultrasonic transducers to measure ultrasonic velocities. There has long been a need for such apparatuses and methods in which a single test of a relatively small amount of a core sample can yield the ultrasonic velocity-mean effective stress ultrasonic velocity-mean effective stress-strength, and ultrasonic velocity-means effective stress-elastic property relationships for a low permeability rock. There has long been a need for triaxial test apparatuses and methods which could effectively, accurately, and rapidly measure ultrasonic velocities while simultaneously measuring pore pressure of an undisturbed low permeability rock sample.